docs: def _get_subparser_line_args(self, subparser_prog):
      """ Get line args of a specified subparser by its prog."""
  
      subparsers = self._get_subparsers()
      for subparser in subparsers:
        if subparser_prog == subparser.prog:
          # Found the subparser.
          args_to_parse = []
          for action in subparser._actions:
            if action.option_strings:
              for argname in action.option_strings:
                if argname.startswith('--'):
                  args_to_parse.append(argname[2:])
          return args_to_parse
  
      return None